Cryogenic air separation plants are typically designed, constructed and operated to meet the baseload product slate demands/requirements for one or more end-user customers and optionally the local or merchant liquid product market demand. Product slate requirements typically include a target volume of high pressure gaseous oxygen, as well as various co-products such as gaseous nitrogen, liquid oxygen, liquid nitrogen, and/or liquid argon. The air separation plant is designed and operated based, in part, on the selected design conditions, including the typical day ambient conditions as well as the available utility/power supply costs and conditions
Changes in customer demand for high pressure gaseous oxygen product from an air separation unit (ASU) plant are common, particularly for certain customers connected via a pipeline to a dedicated ASU plant. For example, steel making customers operating electric arc furnaces typically require a continuously varying high pressure gaseous oxygen demand that can range from basically no flow of high pressure gaseous oxygen to a peak flow greater than the gaseous oxygen capacity of the ASU in a matter of just a few minutes.
To meet these rapidly varying high pressure gaseous oxygen demands, it is desirable to change the ASU plant operating characteristics in order to adjust product flow variations. However, most ASU plants cannot rapidly adjust to the dramatic or extreme fluctuations in gaseous oxygen demand by varying the incoming feed air flow rate as the ASU plant dynamics are typically not fast enough to change operating points or to maintain product purities in these short timeframes. In addition, such extreme fluctuations in high pressure gaseous oxygen demand often lead to extreme operational swings which can adversely impact the reliability and maintainability of the ASU plant equipment, particularly, the compressors and turbo-expanders. Further problems associated with changing high pressure gaseous oxygen demands, rapidly or otherwise, is the impact to the production rate of any ASU plant co-products, such as gaseous nitrogen, liquid nitrogen, liquid oxygen, and argon.
As a result, the most common prior art solution to address the rapid decrease in high pressure gaseous oxygen demands is to have the ASU plant produce the high pressure gaseous oxygen at full capacity and vent any unwanted or unneeded high pressure gaseous oxygen to the atmosphere, while the ASU plant is slowly turned down. In situations, where a rapid increase in gaseous oxygen demand are expected, the ASU plant often continues to produce high-pressure gaseous oxygen at full capacity and without turn-down while continuously venting any excess high pressure gaseous oxygen product. Also, in situations when customer demand for high pressure gaseous oxygen is reduced but there remains a need to maintain production of various co-products, the high pressure gaseous oxygen is often vented incurring the operating cost penalty of venting the high pressure gaseous oxygen without any mitigating benefit.
Examples of the prior art venting of high pressure gaseous oxygen can be found in United States Patent Application publications Nos. 2009/0120129; and US2011/0011130 as well as U.S. Pat. Nos. 5,590,543; and 5,928,408.
Accordingly, there is a need to more quickly respond to rapid changes in high pressure gaseous oxygen demand from an ASU plant while avoiding the operating cost penalty associated with venting of high pressure gaseous oxygen. Ideally, such rapid response would also achieve or facilitate advantages and benefits such as concurrently increasing the argon or liquid nitrogen production from the ASU.